Phase controlled alternating current power circuits
using_ bidirectional conducting devices



April 28, 1970 STORM Re. 26,866

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April 28, 1970 STORM Re. 26,866

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United States Patent PHASE CONTROLLED ALTERNATlNG CURRENT POWER CIRCUITSUSING BIDIRECTION AL CON- DUCTING DEVICES Herbert F. Storm, Delmar,N.Y., assignor to General Electric Company, a corporation of New YorkOriginal No. 3,348,128, dated Oct. 17, 1967, Ser. No. 378,378, June 26,1964. Application for reissue May 6, 1968, Ser. No. 728,089

Int. Cl. Gf U44, U56 US. Cl. 323-4 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE To compensate for the tendency of anunsymmetrical load to saturate an AC supply transformer, a currenttransformer is coupled to the supply transformer for sensing theundesired D-C component and deriving an output control signal pulsewhich is supplied to the firing circuit of a diac or triac bidirectionalconducting device. A compensating D-C component is produced by turningon the device at different phases of the half cycles of the AC supplypotential and is applied to the supply transformer secondary windings bymeans of an auxiliary secondary winding or by connecting the load inseries with the diac or triac and the secondary winding to correct theload current.

This invention relates to phase controlled alternating current powercircuits using bidirectional conducting devices.

More particularly, the invention relates to phase controlled alternatingcurrent power circuits employing single phase supply transformers andbidirectional conducting devices, and wherein saturation eflects of thesupply transformer are minimized.

Many alternating current power circuits employ single phase transformersto supply alternating current electric potential to various branches ofthe circuit. One of the problems associated with the use of single phasesupply transformers in alternating current power circuits arises becauseof the unsymmetrical loading of the transformer as, for example, whenthe load contains direct current components, because of the presence ofa rectifier in the load being supplied by the transformer. Under suchconditions the supply transformer may become saturated at some point inone of the half cycles of the supply alternating current potential. Theeffect of saturation of the supply transformer is the distortion of theresulting output wave shape and higher core losses giving rise to ademand for a larger transformer to overcome these undesired elTects. Asa consequence, for any given alternating current circuit of a statedpower rating, the size and hence, cost of the supply transformeremployed must be considerably increased. To overcome this requirement,the present invention was devised.

The same problem can exist in a variety of electric power circuitconfigurations. For example, a transformer secondary winding that iscoupled to controlled switching means arranged to conduct secondarycurrent during both positive and negative half cycles of the supplyvoltage can be subject to an undesired D-C component as a result 0asymmetrical firing of the switching means.

It is therefore, a primary object of the present invention to provide afamily of new and improved phase controlled alternating current powercircuits employing supply transformers and solid state bidirectionalconducting Re. 26,866 Reissuecl April 28, 1970 "ice [devices] means toovercome saturation effects of the supply transformer.

In practicing the invention a phase controlled alternating current powercircuit is provided which includes a supply transformer and sensingmeans for sensing undesired saturation effects in the supply transformerand, deriving control signal pulses having a polarity, magnitude andphase dependent upon the saturation effects to be corrected. Aconductivity controlled bidirectional conducting device is operativelycoupled to the electric power circuit including the supply transformerand is responsive to the control signal pulses produced by the sensingmeans whereby the phase of the turn-on time of the bidirectionalconducting device is controlled with respect to the supply alternatingcurrent potential in a manner to minimize the saturation effects of thesupply transformer.

Other objects, features and many of the attendant advantages of thisinvention will be appreciated more readily as the same becomes betterunderstood by reference to the following detailed description, whenconsidered in connection with the accompanying drawings, wherein likeparts in each of the several figures are identified by the samereference character, and wherein;

FIGURE 1 is a schematic circuit diagram of a phase controlled powercircuit for a five layer semiconductor conductivity controlled diacbidirectional conducting power device;

FIGURE 2 is a characteristic curve illustrating the voltage versuscurrent operating characteristics of the diac bidirectional conductingpower device;

FIGURE 3 is a series of curves illustrating the voltage versus timeconducting characteristics of the diac bidirectional conducting powerdevice connected in the circuit arrangement of FIGURE 1 for varyingphase angles at which the power diac is rendered conductive with respectto a supply alternating current potential;

FIGURE 4 is a partial schematic circuit diagram of an alternative firingcircuit arrangement for the power diac;

FIGURE 5 is a schematic diagram of still another form of firing circuitarrangement for the diac bidirectional conducting power device used inthe phase control firing circiut of FIGURE 1;

FIGURE 6 is a series of curves illustrating the operatingcharacteristics of the diac bidirectional conducting device whenoperated as a phase controlled half-wave rectifier;

FIGURE 7 is a detailed circuit diagram of a new and improved alternatingcurrent power circuit employing a supply transformer and a diacbidirectional conducting device for minimizing saturation effects in thetransformer;

FIGURE 8 is a detailed circuit diagram of a circuit arrangement similarto that of FIGURE 7 but which employs a triac bidirectional conductingpower device in place of the power diac;

FIGURE 9 is a series of curves illustrating the operation of the currenttransformer for sensing saturation effects in the supply transformer ofFIGURE 7 or 8;

FIGURE 10 is a series of voltage versus time characteristic curvesillustrating the time of firing of the conductivity controlledbidirectional conducting device with respect to an alternating currentsupply potential, and its effect on load current supplied through thedevice;

FIGURE 11 is a detailed circuit diagram of a phase controlledalternating current power circuit employing a supply transformer and apower diac for minimizing saturation effects in the supply transformerwithout requiring modification of the supply transformer itself;

FIGURE 12 is a voltage versus time characteristic curve illustrating theoperating characteristics of the circuit shown in FIGURE 11;

FIGURE 13 is a detailed circuit diagram of a triac version of thecircuit shown in FIGURE 11;

FIGURE 14 is a detailed circuit diagram of a new and improvedalternating current power circuit employing a supply transformer and apower diac bidirectional conducting device for minimizing saturationeffects in the supply transformer without adversely affecting the loadcurrent being supplied to a load;

FIGURE 15 is a series of voltage versus time characteristic curvesillustrating the manner of operation of the circuit arrangement shown inFIGURE 14; and

FIGURE 16 is a detailed circuit diagram of a triac version of thecircuit arrangement shown in FIGURE 14.

The circuit arrangement shown in FIGURE 1 of the drawings is comprisedby a diac bidirectional conducting power device 11 connected in seriescircuit relationship with the small saturable core 12 and a loadresistor 13 across a pair of power supply terminals 14 and 15. The powersupply terminals 14 and 15 in turn are connected across a conventional60 cycle alternating current power source. The diac bidirectionalconducting power device 11 is a five-layer semiconductor NPNPN junctionbidirectional conducting device such as described more fully in anarticle entitled Two-Terminal Asymmetrical and Symmetrical SiliconNegative Resistant Switches by R. W. Aldrich and N. Holonyak, Jr.,appearing in the Journal of Applied Physics, vol. 30, No. 11, November1959, pages 1819-1824. Briefly, however, it can be stated that the diacbidirectional conducting power device 11 can be designed to switch large(power level) currents in the order of 50 to 125 amperes at 250 volts.The power diac can be switched from its nonconducting blocking conditionto its conducting condition by the application of a sharp voltage riseacross its load terminals 1 and 2 to conduct current in either directionthrough the device as determined by the polarity of the appliedalternating current potential supplied to the terminals 14 and 15.

In order to turn-on the power diac bidirectional conducting device 11and render it conductive, the load terminals 1 and 2 of the device areconnected across a pair of firing circuits 16 and 17 respectively. Thefiring circuit 16 is comprised by a current limiting resistor 18, afirst set of relay operated contacts 19 connected in series circuitrelationship with a parallel charging network comprised by a capacitor21, a resistor 22, and a battery 23 or other suitable source of directcurrent electric potential. By this arrangement, the battery 23 willcharge the capacitor 21 to essentially the full potential of thebattery, and upon the contacts 19 being closed, capacitor 21 will serveto drive the potential of the load terminal 2 of diac 11 sharplypositive with respect to the load terminal 1. Under these conditions,and assuming that the terminal 14 at that instant is positive withrespect to terminal 15 then load current will be conducted through diac11 in the direction of the solid arrow i;,.

The firing circuit 17 is further comprised by a limiting resistor 25 andsecond set of relay operated contacts 26 connected in series circuitrelationship with a parallel connected capacitor 27, resistor 28, andbattery 29. It should be noted that the battery 29 or other source ofdirect current electric potential has its polarity reversed with respectto the battery 23. As a consequence, upon the relay contact 26 beingclosed, capacitor 27 will drive the potential of the load terminal 2 ofdiac 11 sharply negative with respect to terminal 1 of power diac 11. Asa result, and assuming the terminal 15 is now positive with respect toterminal 14, load current will be conducted through diac 11 in thereverse direction shown by the dotted arrow. The contacts 19 and 26 areactuated by their associated relay windings 31 and 32, respectively, towhich they are mechanically connected by a solenoid actuated push rod orthe like. The relay windings 31 and 32 are in turn energized from aselsyn generator 33 which is supplied from the same 60 cycle alternatingcurrent supply connected to the supply terminals 14 and 15. Theenergizing potential supplied from the selsyn generator 33 is applied tothe relay winding 31 through a variable resistor 34 and diode 35, andthe relay winding 32 similarly is energized through a. variable resistor36 and associated diode 37. By this arrangement, relay contacts 19 and26 can be operated synchronously with the alternating current supplyvoltage applied across the terminals 14 and 15, or by varying the valuesof the resistors 34 and 36, varying degrees of phase retardation can beintroduced into the time at which the relay contact 19 or 26 close tothereby introduce a phase retardation into the time of conduction of thediac bi directional conducting power device 11.

FIGURE 2 of the drawings illustrates the voltage versus currentcharacteristics of the diac bidirectional conducting device 11 whereinit can be seen that the device can be caused to conduct currentirrespective of the polarity of the voltage applied across the device.

FIGURE 3 of the drawings illustrates a series of voltage versus timecharacteristic curves wherein varying amounts of phase retardation areintroduced into the point at which the bidirectional conducting powerdiac device is rendered conductive with respect to [the phase] a cycleof the alternating current supply potential. In these figures, the uppertrace shows the voltage appearing across the power diac device 11, andthe lower trace illustrates the load current i flowing through thedevice. Accordingly, it can be appreciated that where no phaseretardation is introduced into the time of firing the diac, there ispractically zero voltage continuously across the diac 11, and a fullwave of load current flows through the load resistor 13 as shown inFIGURE 3(a) of the drawings.

In FIGURES 3(b) through 3(g) varying amounts of phase retardation areintroduced into the time of firing of the power diac 11. From anexamination of these curves it can be appreciated that correspondinglyvarying amounts of voltage appearing across the diac, and varying valuesof load current are caused to flow in the circuit arrangement ofFIGURE 1. Specifically, FIGURE 3(b) illustrates a condition for phaseretardation of five degrees; FIGURE 3(c) shows a phase retardation of45; FIGURE 3(d) illustrates the condition for a phase retardation ofFIGURE 3(c) shows the voltage and load current for phase retardation ofFIGURE 3(f) for a and FIGURE 3(g) for Those skilled in the art refer tothe degrees of phase retardation as a firing angle. From a comparison ofFIGURE 3(a) and FIGURE 3(g), it can be appreciated that the circuit goesfrom a condition of essentially full voltage across the load and novoltage across the diac device 11, to a condition where there isessentially full voltage across the diac device, and no voltage acrossthe load. It should also be noted that essentially continuous controlcan be obtained from the 0 to 180 phrase retardation condition withoutany instability. During turn on, the small saturable reactor 12 holdsoff the firing potential applied across terminals 1 and 2 for only ashort time period to cause a sharp increase in voltage across power diac11 to thereby cause it to turn on more quickly than if the reactor 12were not present.

With one of the relay coils 31 or 32 disconnected, the diacbidirectional conducting device 11 of FIGURE 1 will operate as a phasecontrolled rectifier. Operation of the circuit of FIGURE 1 in thismanner is illustrated in FIGURE 6 of the drawings wherein FIGURE 6(a)illustrates the voltage across the diac 11 is zero from 0 to 180 andappears from 180 to 360 where there is no phase retardation introducedinto the time of firing of the diac. As a consequence, load current willflow for onehalf cycle as in a rectifier. FIGURES 6(b) through 6(g)illustrate the condition for varying amounts of phase retardationintroduced into the time of firing of the diac 11 wherein it can be seenthat the current wave shapes are similar to those that can be obtainedwith a conventional SCR using phase control. If the other one of the tworelay windings 31 or 32 were disconnected so that the alternate voltagespike were used to turn on the diac 11, a similar control would beachieved except that the load current flow would be in the oppositedirection. Accordingly, it can be appreciated that if the load resistor13 were replaced by the armature of a shunt field excited motor, onecould not only control the speed of the motor by controlling the firingangle, but could also control the direction of rotation by supplyingfiring spikes of one direction or the other to the power diac 11.

FIGURES 4 and 5 of the drawings illustrate alternative arrangements forfiring power diac 11. In the circuit arrangement shown in FIGURE 4, acoupling transformer 41 is used which has its secondary winding 43connected across the diac 11 through a coupling capacitor 45, and itsprimary winding could be connected to a phase control circuit such asthat shown in FIGURE 1 by connection between the juncture of thelimiting resistors 25 and 18 and the terminal 15. In the circuitarrangement of FIGURE 5, the secondary winding 43 of the couplingtransformer is connected in series circuit relationship with the diac 11and load resistor 13 with a capacitor 45 being connected in parallelcircuit relationship with the series connected diac 11 and the secondarywinding 43. In these arrangements the capacitor 45 provides as direct apath as possible across the terminals of the diac 11 for the voltagespikes used to fire the diac.

FIGURE 7 of the drawings illustrates a practical operating circuitemploying a phase controlled power diac bidirectional conducting device11. The circuit of FIG- URE 7 comprises a new and improved alternatingcurrent power circuit employing a supply transformer and means(including a power diac) for preventing saturation of the supplytransformer during one-half cycle of an applied alternating currentsupply potential. The circuit arrangement of FIGURE 7 is comprised by analternating current supply transformer 51 having [its primary winding 52connected across a source of alternating current potential] a set ofinductively coupled primary and secondary windings 52 and 53,respectively. The primary winding 52 is connected to the alternatingvoltage supply lines 14 and 15 as shown. The secondary winding 53 of thesupply transformer 51 is connected across a load 54 connected in seriescircuit relationship with a rectifier 55 which together operate as anunsymmetrical load on the secondary winding 53. As a result of theunsymmetrical nature of the load 54 and 55, the supply transformer 51will tend to saturate during one-half cycle of the applied alternatingcurrent potential. This is caused by the direct current componentflowing in the secondary winding 53 as a result of the presence of therectifier 55, which in effect causes the secondary winding 53 to beunsymmetrically loaded.

FIGURE 9 of the drawings illustrates the nature of the phenomenoninvolved. In FIGURE 9(A) the voltage versus time characteristic of theapplied alternating current potential is plotted. FIGURE 9(B)illustrates the nature of the magnetizing current which flows in thesecondary winding 53 due to the saturation effect during onehalf cycleof the applied alternating current potential. This magnetizing currentis the difference between the currents flowing in the primary winding 52and the secondary winding 53, and for the circuit arrangement shown inFIGURE 7 results in the current peaks shown in FIGURE 9(B) in solidlines. In the event of the reversal of the connection of the dioderectifier 55, saturation of the supply transformer would take place onthe other side of the hysteresis loop of its core, and the wave shape ofthe curve shown in FIGURE 9(B) would be shifted 180, and its polarityreversed as illustrated by the dotted curve in FIGURE 9(B). Accordingly,it can be appreciated that the position of the current peaks in FIGURE9(B) and their polarity provide one with information as to which side ofthe hysteresis loop of the transformer core the transformer issaturating, and the height or magnitude of the peak provides one withthe information of the degree of saturation.

In order to derive the above described information, a

current transformer sensing means 56 is provided which includes the twoprimary windings 57 and 58 and output winding 59. The primary winding 57is connected in series circuit relationship with the primary winding 52of supply transformer 51, and the primary winding 58 is connected inseries circuit relationship with the secondary winding 53 of supplytransformer 51. By placing these two windings on a common core andproperly proportioning the turns ratios of the two primary windings 57and 58, the current i; induced in the output winding 59 will be areplica of the magnetizing current tending to drive the supplytransformer 51 into saturation. This current is the difference betweenthe current flowing in the primary winding 52 and the current flowing inthe secondary winding 53. The magnetizing current i, is illustrated inthe solid line curves of FIGURE 9(B). It, of course, follows that if theopposite side of the hysteresis loop of the core of supply transformer51 is being saturated, then the current i would be as shown in thedotted curve of FIGURE 9(B).

The replica of the magnetizing current i, is derived by the currenttransformer sensing means 56 is then supplied to a potentiometercomprised by variable resistor 61 where it develops a voltage that isapplied to an avalanche operated conductivity controlled bidirectionalconducting device 62. The avalanche operated conductivity controlledbidirectional conducting device 62 may comprise any conventional snapswitch device such as a three layer signal diac being manufactured andsold by the General Electric Company, Rectifier Components Department,Auburn, N.Y., a Hunt diode manufactured and sold by the Hunt ElectricManufacturing Company, or a bidirectional conducting Shockley diode. Allof these devices are voltage sensitive devices which can break down andconduct current in either one of two directions upon the voltage acrossthe device rising to a predetermined firing level, and upon the voltageacross the device dropping below a predetermined holding level, thedevice reassumes its blocking condition. By proper adjustment of thesetting of the variable resistor 61, one can control the potential atwhich the conductivity controlled bidirectional conducting device 62breaks down and conducts. Hence, device 62 serves as a clipping devicewhich develops a clipped output current i that is supplied to a parallelconnected capacitor 63 and resistor 64 connected across the avalancheoperated bidirectional conducting device 62. As a result of thisarrangement, a voltage e will be developed across the capacitor 63 whichis illustrated in FIGURE 9(D) of the drawings by the solid line curve.For more intense saturation conditions of the supply transformer 51, thevoltage e will become larger. If the supply transformer 51 saturates inthe opposite direction, that is, it saturates on the opposite side ofits hysteresis loop, the olarity of the voltage e will be reversed andwill be phase shifted by as illustrated in the dotted curve shown inFIGURE 9(D).

The capacitor 63 is coupled across the primary winding 65 of a pulsetransformer 66 whose split secondary Winding halves 67 and 68 areconnected to the base electrodes of a pair of NPN junction transistors69 and 71, respectively. The transistors 69 and 71 are connected in aconventional alternating current push-pull amplifier arrangement, andfor this purpose, the emitter electrodes of the transistors 69 and 71are connected in common to a mid tap point on the secondary windings 67,68 of pulse transformer 66 and the collector electrodes of the NPNjunction transistors 69 and 71 are connected across a primary winding 72of an output pulse transformer 73. Operating potentials are supplied tothe amplifying means comprised by transistors 69 and 71 from a rectifierpower supply comprised by a limiting resistor 74, a rectifier 75 andassociated filter network including an inductance 76 and capacitor 77connected across the alternating current power supply terminals 14 and15. In order to provide rectified direct current bias potentials totransistors 69 and 71, a limiting resistor 74 is connected to a mid tappoint on the output winding 72 of pulse transformer 73, and the emittersof transistors 69 and 71 are connected in common to the power supplyterminal which if desired may be grounded. By reason of thisarrangement, the voltage pulses e which are representative of theundesired magnetizing current flowing in the supply transformer 51 areamplified by push-pull amplifier 69 and 71, and then supplied throughoutput coupling transformer 73 where they are applied across thebidirectional conducting power diac device 11 by the secondary winding78 of output pulse transformer 73.

The bidirectional conducting power diac device 11 is operativelyconnected in series circuit relationship with a small saturable corewinding 12 and a load resistor 81 across the secondary winding 53 of thesupply transformer 51. The power diac device 11 is turned on by a firingcircuit means which includes the secondary winding 78 of output pulsetransformer 73, a charging capacitor 82, and firing circuit meanscomprised by a second ava lanche operated bidirectional conductingdevice 83 which is similar in nature to the avalanche operatedbidirectional conducting device 62. The capacitor 82, secondary winding78, and avalanche operated bidirectional conducting device 83 are allconnected in series circuit relationship across the diac bidirectionalconducting device 11 at a point intermediate the diac 11 and the smallsaturable reactor winding 12. By this arrangement, current pulsesinduced in the secondary winding 78 of output pulse transformer 73 willcause the diac bidirectional conducting device 11 to be turned on andrendered conductive in a manner that will be discussed more fullyhereinafter.

Upon the diac bidirectional conducting device 11 being turned on andrendered conductive, current flows in the load resistor 81 which will inturn develop a current flow through a small auxiliary winding 85connected in series circuit relationship with a choke coil 86 across theload resistor 81. The small auxiliary Winding 85 is wound on the core ofthe supply transformer 51 in common with the primary winding 52 andsecondary winding 53 in a manner such that the ampere turns induced bythe small auxiliary winding 85 oppose the ampere turns of the secondarywinding 53. By reason of this arrangement, a direct current component ofa proper phase, polarity and magnitude is induced in the small auxiliarywinding 85 which is in such a direction as to drive the core of thesupply transformer 51 out of saturation thereby obviating any saturationeffects in the transformer.

The manner in which the compensating D-C component is developed in thesmall auxiliary winding 85 can best be appreciated in connection withFIGURE 10 of the drawings. As shown in FIGURE 10(A), the voltage eappearing across the diac bidirectional conducting device 11 isessentially the same as the 60 cycle alternating current supplypotential. Ignoring for the moment the effect of the gating pulsesupplied to the secondary winding 78, then by reason of this potentialthe rise in potential of the capacitor 82 indicated at e in FIGURE 10(A)will lag the supply potential e due to the drop in resistor 84, an disadjusted to attain the value e, at a phase position of 90. The value eis the firing potential of the avalanche operated bidirectionalconducting device 83 so that at this point, a firing pulse will beapplied across the diac bidirectional conducting device 11. Upon afiring pulse being applied across the diac bidirectional conductingdevice 11, the load terminal 2 of the device will be driven sharplypositive With respect to the load terminal 1. It should be noted thatduring this firing interval the small saturable reactor 12 will hold offthe firing potential sufficiently to give rise to this sharp rise inpotential of the load terminal 2 thereby causing the diac 11 to berendered fully conductive almost instantaneously. As a consequence ofdiac 11 being rendered conductive, the voltage e across the diac dropsessentially to zero as shown in FIGURE 10(A) and the voltage across theload resistor 81 shown as e;, in FIGURE 10(B) will rise sharply toessentially the full potential of the alternating current power supply.During the succeeding half cycle of the supply alternating currentpotential, substantially the same process will occur but in a reversepolarity sense to develop across the load resistor 81 an alternatingcurrent potential having the wave shape :2 shown in FIG- URE 10(B).

Consider now the effect of the current pulse applied to the capacitor 82by output pulse transformer 73. If capacitor 82 were biased with anegative charge at the time t:0, it can be appreciated from anexamination of FIG- URE 10(C) that it will take more time for thecapacitor voltage c to reach the diode breakdown firing voltage o incomparison to the time required to reach the same voltage under thecircumstances plotted in FIGURE 10(A). Hence, the time of firing of thediac 11 during the positive half cycle of the applied alternatingcurrent potential will be delayed in the manner shown in FIGURE 10(C).The result is a reduction in the average value of the load voltage eduring the positive half cycle of the supply voltage from 0 to 180 asshown in FIGURE 10(B). If no bias is applied to the capacitor 82 duringthe following negative half cycle of the supply voltage from 180 to 360,the load voltage o will remain the same as shown in FIGURE 10(B). Itfollows, therefore, that the biasing of the capacitor 82 by the currentsupplied from pulse transformer 73 results in a load voltage e which nowhas a direct current component. If the capacitor 82 is biased with alarger charge, the direct current component will also be larger.Further, if the capacitor 82 is biased with a reversed polarity chargethat is phase displaced by 180 electrical degrees, the direct currentcomponent in the resulting load voltage e will also have the oppositedirection and a magnitude dependent upon the magnitude of the reversepolarity phase displaced charge. It can be appreciated, therefore, thatby feeding back the direct current component appearing across the loadresistor 81 to the auxiliary winding 85, a corrective current is causedto flow in the auxiliary winding 85 which is in such a direction as toprevent saturation of the supply transformer core. The choke coil 86connected in series with the auxiliary winding 85 serves to suppress anyaltemating current induced in the auxiliary Winding 85 by thetransformer core of the supply transformer 51.

FIGURE 8 of the drawings illustrates a circuit arrangement similar tothat shown in FIGURE 7 wherein a triac bidirectional conductivitycontrolled conducting device 91 is substituted for the diacbidirectional conducting device 11. The triac bidirectional conductingdevice 91 is a new triode A-C semiconductor switch which can beperiodically triggered into conduction by a gate signal applied to itscontrol gate in a manner similar to the action of a conventional siliconcontrolled rectifier. The triac differs from the conventional SCR,however, in that it can conduct current in both direcitons dependingupon the polarity of the potential across the load terminals of thedevice, and gating-on can be accomplished in response to either apositive or negative gate signal. A triac is essentially a five layerNPNPN junction semiconductor device which is manufactured and sold bythe Rectifier Components Dept. of the General Electric Co., located inAuburn, N.Y. For a more complete description of the triac. reference ismade to Application Note No. 200.35 issued February 1964 entitled TriacControl for A-C Power" by E. K. Howell, published by the RectifierComponents Dcpt. of the General Electric Company at Auburn, N.Y.Reference may also be made to an article entitled, Bilaterial SCR LetsDesigners Economize on Circuitry, by E. K. Howell appearing inElectronic Design Magazine, Jan. 20, 1964, issue.

By reason of its novel gate firing characteristic, the triac 91 may haveits gate electrode connected directly to the firing circuit meanscomprised by avalanche operated bidirectional gating device 83. Gatingdevice 83 in turn is connected directly across the charging capacitor 63with the capacitor 63 being connected in series circuit relationshipwith a limiting resistor 84 across the load terminals of the triacdevice 91. By reason of this arrangement, the need for the push-pullamplifying means comprised by transistors 69 and 71, their associatedcoupling transformers, and the direct current power supply connectionsis eliminated. As a consequence, a much simpler circuit configurationresults since the voltage e used to file the triac 91 can now be thevoltage e appearing across the capacitor 63. In all other respects, thecircuit will function in precisely the same manner as the circuitarrangement of FIGURE 7 to develop a direct current component in theload voltage developed across the resistor 81. This direct currentcomponent in turn supplies the compensating arnpere-turns to theauxiliary winding 85 to unsaturate the supply transformer 51. Since themanner in which such operation is achieved was discussed fully inconnection with the circuit arrangement of FIGURE 7, a further detaileddescription of the manner of operation of the circuit of FIGURE 8 isbelieved unnecessary. It should also be noted that while the circuit ofFIGURE 8 is simpler in construction, the triac 91 is inherently a slowerresponding device than diac 11 and hence the circuit of FIGURE 7 may bedesigned to be faster responding than the circuit of FIGURE 8.

With the circuit arrangements of FIGURES 7 and 8, it is necessary tomodify the construction of the supply transformer 51 to incorporate theauxiliary winding 85 around the core thereof. Such modification of thesupply transformer may not be possible in certain circumstances, andhence use of the circuit arrangements of FIGURES 7 and 8 will beinappropriate. The circuit arrangement of FIGURE 11 is designed for usein those circumstances where it is not appropriate to modify theconstruction of the supply transformer either because of economy, orbecause the nature of the load prevents it. The circuit shown in FIGURE11 is substantially similar to the circuit arrangement of FIGURE 7 withthe exception that the load 92 is connected in series circuitrelationship with a diac bidirectional conducting device 11 and smallsaturable reactor winding 12 across the secondary Winding 53 of thesupply transformer 51. By connecting the load 92 in this manner, it ispossible to correct the load current applied to load 92 itself so as tocompensate for saturation effects in the supply transformer 51. Themanner in which this is accomplished with the circuit arrangement ofFIGURE ll can best be appreciated in connection with FIGURE 12 of thedrawings.

FIGURE 12(a) illustrates the voltage wave shape of the appliedalternating current supply potential e appearing between terminals 14and 15. Curve e in FIGURES 12(b) and 12(c) represents the voltage riseacross the diac bidirectional conducting device 11 over a full cycle ofthe applied alternating current supply potential. Curve e represents thevoltage rise across the capacitor 82 assuming that there is noadditional charge applied thereto from the secondary winding 78 of pulsetransformer 73. It should be noted that under such condition the voltageon capacitor 82 will rise sufficiently to the firing voltage e; ofdevice 83 to fire the diac bidirectional conducting device 11 atsubstantially 90 phase relation with respect to the supply potentialduring the positive half cycle, and at 270 phase relation during thenegative half cycle. As a result, the load voltage supplied to load 92would have the wave shape e shown in FIGURE 12(b).

In the event that the core of the supply transformer 51 is driven intosaturation during either half cycle of the applied alternating currentsupply potential, the current transformer sensing means 56, avalancheoperated bidirectional conducting device 62, and amplifying circuitmeans 69, 7|] will develop control signal pulses of the nature shown inFIGURE 9(c) which are supplied to the capacitor 82 through output pulsetransformer 73. These control current pulses will bias the capacitor 82in the manner shown in FIGURE 12(c) of the drawings so that thepotential across the capacitor e will require a greater length of timeto reach the firing potential c hence, a greater portion of the positivehalf cycle of the alternat ing current supply potential is required toreach the firing potential e; of the avalanche operated bidirectionalconducting device 83. Upon reaching this level, the diac 11 will, ofcourse, be triggered on and rendered conductive in the previouslydescribed manner to supply load current of positive polarity to the load92. This load current will have a wave shape such as is illustrated at ein FIGURE 12(c) of the drawings wherein it can be appreciated that therewill be developed a compensating direct current component which willtend to unsaturate the core of the alternating current supplytransformer 51.

FIGURE 13 is of the drawings illustrates a form of the circuitarrangement shown in FIGURE 11 wherein [a triac bidirectional conductingdevice 91] conductivity controlled solid state switching means, such asthe illustrated triac 91, is employed in place of the diac 11. Becausethe triac 91 can be turned on with only a small value gate turn-onsignal, it is possible to eliminate the amplifying mean comprised bytransistors 69 and 70, and apply the control signal pulses developedacross the capacitor 63 directly to the charging capacitor 82 throughthe medium of a pair of voltage dividing resistors 84 and 93 connectedin series circuit relationship with the charging capacitor 82 across theterminals of the secondary winding of supply transformer 51. The effectof connection of the circuit in this manner will be to apply a biascharge to the charging capacitor 82 in the manner depicted in FIGURE12(c) of the drawings so that [the triac 91 will be turned on at a pointwith respect to the phase of the applied alternating current supplypotential] the firing angle at which the triac 91 commences to conductin the positive" direction during one half cycle of the alternatingvoltage is shifted relative to the firing angle at which it commences toconduct in the "negative" or opposite direction during the succeedinghalf cycle in order to develop the desired load current as representedby the load voltage wave shape e illustrated in FIGURE 12(c). As shown,the shift in firing angles efiects difieren! angles for positive andnegative half cycles; alternatively, if the saturation tendency were dueto an unintentional disparity between the respective firing angles, itwould restore symmetry. As a consequence of operation of the triac 91 inthis manner, a compensating direct current component will be developedin the secondary winding 53 of transformer 51 which will tend tounsaturate the transformer thereby obviating the undesired saturationeffects. It should be noted while the circuit arrangement of FIGURE 13is considerably simpler from the circuit arrangement of FIGURE 11, thespeed of response of the circuit arrangement of FIGURE 11 is greaterthan that of FIGURE 13 arrangement due to the fact that the diac 11 isan avalanche operated device which is rendered conductive almostinstantaneously across its entire cross section. Thi is in contrast tothe gate controlled triac device 91 which requires a finite time topropagate carriers of conduction across the cross section of the devicethereby limiting its speed of response.

From an examination of curve 12(c) of the drawings, it will beappreciated that in order to derive the compensating direct currentcomponent for unsaturating the supply transformer 51 with the circuitarrangements of FIGURES 11 and 13, it is necessary to vary the value ofthe load current supplied to load 92. By this is meant that the rootmean square value of the load current is varied so that in order toutilize the circuit arrangements of FIGURES 11 and 13 it is necessarythat the load 92 be able to accommodate such variations in the value ofthe load current supplied to it. Since there may be certain applicationswhere it is not possible to tolerate such variation of the root meansquare value of the load current, the circuit arrangements of FIGURES l4and 16 were derived wherein the root mean square value of the loadcurrent is maintained constant. Nevertheless, with the circuitarrangements of FIGURES 14 and 16, a direct current component is derivedwhich prevents saturation of the supply transformer 51 and maintains thesupply transformer 51 operating in an unsaturated condition.

To accomplish the above-mentioned end, the circuit arrangements ofFIGURES 14 and 16 have been designed to include a circuit means formaintaining the bias on the charging capacitor 82 throughout a fullcycle of the applied alternating current supply potential. For thispurpose. the circuit ararngement of FIGURE 14 supplies the controlsignal pulse developed by the current transformer sensing means 56through the avalanche operated bidirectional conducting device 62 to arectifier bridge arrangement 101. The output from the rectifier bridge101 is coupled through a coupling diode 102 to a tank circuit comprisedby a parallel connected capacitor 103 and inductor 104. The inductor 104comprises the primary winding of a coupling transformer 105 whose splitsecondary winding halves 106 and 107 are connected in circuitrelationship with four NPN junction transistors 108. 109, 111 and 112 toform a phase discriminator. Alternating current power is supplied to thephase discriminator from a supply transformer 113 whose primary winding114 is connected across the secondary winding 53 of supply transformer51, and whose split secondary winding halves 115 and 116 are connected,respectively, between the collector electrodes of transistors 108 and109, and between the collector electrodes of transistors 111 and 112,respectively. Because the phase discriminator comprised by the windings106, 107, transistors 108 through 112 and windings 115 and 116 is aconventional circuit which has been described in detail in a number oftextbooks on electronic circuits, a detailed description of itsconstruction and manner of operation is believed unnecessary. It isbelieved adequate to point out that the phase discriminator circuit106-116 will develop across a load resistor 117 connected to the outputof the phase discriminator, a bias current whose polarity, magnitude andphase relation is determined by the polarity, magnitude and phaserelation of the control signal pulses derived from the output of thecurrent transformer sensing means 56. The load resistor 117 is connectedin series circuit relationship with a capacitor 63, and a variableresistor 118 across the secondary winding of the supply transformer 51so that the charge on the capacitor 63 is determined by the combinationof the alternating current supply potential, and the bias derived fromthe load resistor 117.

Capacitor 63 is connected through a second avalanche operatedbidirectional conducting device 119 which is similar in nature to theavalanche operated bidirectional conducting device 62, and is connectedacross primary winding 65 of the input transformer 66 to a push-pullamplifying means comprised by the transistors 69 and 70. The output ofthe amplifying means 69 and 70 is supplied through an output couplingtransformer 73 and appears across the secondary winding 78 of thetransformer. The secondary winding 78 is connected in series circuitrelationship with a charging capacitor 82 and limiting resistor 84across the secondary Winding of the supply transformer 51. The junctureof the bias winding 78 and limiting resistor 84 is connected through athird avalanche operated device 83 to the load terminal 2 of the diacbidirectional conducting device 11. The diac bidirectional conductingdevice 11 is connected in series circuit relationship with the smallsaturable reactor winding 12 and the load 92 across the secondarywinding 53 of supply transformer 51.

The operation of the circiut arrangement shown in FIGURE 14 can best hedescribed in connection with the voltage wave shapes shown in FIGURE ofthe drawings. In FIGURE 15(a), the alternating current supply potentiale is illustrated. If there is no bias potential supplied from thecurrent transformer sensing means 5-6, the charge on capacitor 82 wouldbe represented by the curve e in FIGURE 15(b) and would normally causethe diac 11 to be turned on at and 270' in one cycle of the alternatingcurrent supply potential in the same manner shown with the curve inFIGURE 12(b) of the drawings. However, in the event that the currenttransformer sensing means 56 derives output current signal pulsesrepresentative of the magnetizing current flowing in the secondarywinding 53 of supply transformer 51, these cur rent signal pulses willdevelop across the load resistor 117 a bias potential which will besupplied through the amplifying means 69 and 70 to the secondary winding78 of the output pulse transformer 73. Because of the oscillatory natureof the tank circuit 103, 104, the bias potential developed across loadresistor 117 will be maintained continuously throughout one entire cycleof the supply alternating current potential e The effect of maintainingthe bias charge on capacitor 63 throughout one entire cycle in thismanner is best illustrated in FIGURE 15(0) of the drawings wherein itcan be seen that because of the bias charge on 63 during the positivehalf cycle of the supply alternating current potential, a longer periodof time will be required for the charge on the capacitor 63 (and hencethe charge on capacitor 82), to reach the firing potential +e; of theavalanche operated bidirectional conducting device 119 and 83 to turn ondiac 11. During this half cycle, the operation of the circuit will besimilar to the operation of the circuit previously described. However,during the negative half cycle of the supply alternating currentpotential, the charge on the capacitors 63 and 82 again will be biasednegative to the value represented by e so that during the negative halfcycle a lesser period of time will be required for the charge on thecapacitor to reach the negative firing potential e; therebyproportionately increasing the load current value during the negativehalf cycle of the supply alternating current potential in the mannershown by the curve e in FIG- URE 15(c) of the drawings. As aconsequence, it can be appreciated that while the compensating directcurrent component is still developed in the load current circuit, thetotal load current flowing through the load 92 is maintained constant.That is to say the root mean square value of the load current ismaintained constant because where the load current is decreased duringthe positive half cycle in order to derive the desired compensatingdirect current component, it is proportionally increased during thefollowing negative half cycle to thereby maintain the root mean squarevalue of the load current constant over a full cycle of the alternatingcurrent supply potential. It should also be noted that while a tunedoscillatory circuit has been illustrated as the means for maintainingthe charge on capacitor 63 (and hence 82) over a complete cycle of thealternating current supply potential, other similar means might beemployed. For example, a bank of various direct current voltage levelscould be selectively switched into circuit relationship with thecapacitor 63 by the control signal pulses from the current transformer.Other equivalent circuit arrangements are believed to be equallyobvious.

FIGURE 16 of the drawings is a triac version of the circuit arrangementof FIGURE 14 wherein the triac bidirectional conducting device 91 issubstituted in place of the diac 11. As a consequence, of thissubstitution. the circuit arrangement can be considerably simplified byelimination of the amplifying circuit means and its associated directcurrent power supply as well as elimination of one of the avalancheoperated bidirectional conducting switches 119. In other respects, theoperation of the circuit of FIGURE 16 is identical to that describedwith relation to the circuit of FIGURE 14 and hence will not be againdescribed in detail.

From the foregoing description, it can be appreciated that the inventionprovides a family of new and improved phase controlled alternatingcurrent power circuits employing supply transformers and solid state[bidirectional conducting devices which are utilized to overcome]switching means whose control means is constructed and arranged to senseand automatically to counteract saturation eflects of the supplytransformer. As a consequence, the need for over designed supplytransformer is obviated.

Having described several embodiments of the new and improved phasecontrol alternating current power circuit constructed in accordance withthe invention, it is believed obvious that other modifications andvariations of the invention are possible in light of the aboveteachings. It is, therefore, to be understood that charges being made inthe particular embodiments of the invention described which are Withinthe full intended scope of the invention are defined by the appendedclaims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An alternating current power circuit including in combination asupply transformer having an undesired direct current component flowingin the secondary winding thereof during one phase of an alternatingcurrent supply potential applied to the supply transformer, currentsensing means comprising a current transformer operatively coupled tothe supply transformer for sensing the undesired direct currentcomponent and deriving an output control signal which has a polarity,magnitude, and phase dependent upon the polarity, magnitude, and phaseof the undesired direct current component flowing in the secondarywinding of the supply transformer, a conductivity controlled solid statebidirectional conducting device operatively coupled with the secondarywinding[s] of the supply transformer, firing circuit means thereforresponsive to the control signal derived from said sensing means forcyclically turning on the bidirectional conduct ing device at differentphases of the positive and negative half cycles of the alternatingcurrent supply potential to thereby derive a compensating direct currentcomponent for minimizing the undesired direct current component flowingin the secondary winding of the supply transformer, and means forapplying said compensating direct current component to the supplytransformer.

2. The combination set forth in claim 1 wherein the last-mentioned meanscomprises an auxiliary winding inductively coupled to at least thesecondary winding of the supply transformer and connected in circuitrelationship with the conductivity controlled bidirectional conductingdevice for supplying ampere turns to the core of the supply transformerin a direction to unsaturate the core of the supply transformer tothereby obviate the effects of the undesired direct current componentflowing in the secondary Winding.

3. The combination set forth in claim 1 wherein to provide thelast-mentioned means the conductivity controlled bidirectionalconducting device is operatively connected in series circuitrelationship with the load to be supplied across the secondary windingof the supply transformer and wherein the load current itself iscontrolled by controlling the phase of the turn-on of the bidirectionalconducting device with respect to the supply alternating currentpotential to thereby obviate the effects of the undesired direct currentcomponent flowing in the secondary winding.

4. An alternating current power circuit including in combination asupply transformer having an undesired direct current component flowingin the secondary winding thereof during one phase of an alternatingcurrent supply potential applied to the supply transformer, currentsensing means comprising a current transformer operatively coupled tothe supply transformer for sensing the undesired direct currentcomponent and deriving an output control signal pulse which has apolarity, magnitude and phase dependent upon the polarity, magnitude,and phase of the undesired direct current component flowing in thesupply transformer when measured with respect to the supply alternatingcurrent potential applied to the supply transformer, an avalancheoperated conductivity controlled bidirectional conducting deviceoperatively coupled to the current transformer sensing means fordeveloping control signal pulses of a given magnitude or greater, signalpulse amplifying means operatively coupled to the output of theavalanche operated conductivity controlled bidirectional conductingdevice, a diac bidirectional conducting device operatively coupled withthe secondary windings of the supply transformer, firing circuit meanstherefor coupled to the output of the signal pulse amplifying means andthereby responsive to the control signal pulses derived from saidsensing means for cyclically turning on the diac bidirectionalconducting device at different phases of the positive and negative halfcycles of the alternating current supply potential to thereby derive acompensating direct current component for minimizing the undesireddirect current component flowing in the secondary winding of the supplytransformer, and means for applying said compensating direct currentcomponent to the supply transformer comprising an auxiliary windinginductively coupled to at least the secondary winding of the supplytransformer and wound on the same core therewith and operativelyconnected to the diac bidirectional conducting device for applyingampereturns to the core of the supply transformer in a direction tounsaturate the core of the transformer to thereby obviate the undesiredeffects of the undersired D-C component flowing in the secondary windingthereof.

5. An alternating current power circuit including in combination analternating current supply transformer having an undesired directcurrent component flowing in the secondary winding thereof which tendsto saturate the core of the transformer in one or the other half cyclesof the alternating current supply potential, current transformer sensingmeans operatively coupled to the supply transformer for sensing theundesired direct current component and deriving output control signalpulses having a polarity, magnitude and phase dependent upon thepolarity, magnitude, and phase of the undesired direct current componentflowing in the secondary winding of the supply transformer, an avalancheoperated conductivity controlled bidirectional conducting deviceoperatively coupled to the output of the current transformer sensingmeans for deriving output control signal pulses of a given magnitude orgreater, a triac bidirectional conducting device operatively coupledwith the secondary windings of the supply transformer, firing circuitmeans therefor responsive to the control signal pulses derived from theoutput of said avalanche operated conductivity controlled bidirectionalconducting device for cyclically turning on the triac bidirectionalconducting device at different phases of the positive and negative halfcycles of the alternating current supply potential to thereby derive acompensating direct current component for minimizing the undesireddirect current component flowing in the secondary Winding of the supplytransformer, and means for applying said compensating direct currentcomponent to the circuit comprising an auxiliary winding on the core ofthe supply transformer and operatively connected in circuit relationshipwith the triac bidirectional conducting device for supplyingampere-turns to the core of the supply transformer to unsaturate thecore and thereby obviate the undesired effects of the undesired directcurrent component flowing in the secondary winding of the supplytransformer.

ti. An alternating current power circuit including in combination analternating current supply transformer having an undesired directcurrent component flowing in the secondary winding thereof during onephase of an alternating current supply potential applied to the supplytransformer, current transformer sensing means operatively coupled tothe alternating current supply transformer for deriving output controlsignal pulses having a polarity, magnitude and phase dependent upon thepolarity, magnitude, and phase of the undesired direct current componentflowing in the secondary winding of the supply transformer, an avalancheoperated conductivity controlled bidirectional conducting deviceoperatively coupled to the output of the current transformer sensingmeans for deriving output control signal pulses of a given magnitude orlarger, signal amplifying means operatively coupled to the output ofsaid avalanche operated conductivity controlled bidirectional conductingdevice for amplifying the control signal pulses, a diac bidirectionalconducting device operatively connected in series circuit relationshipwith the secondary windings of the alternating current supplytransformer, firing circuit means therefor coupled to the output of thesignal amplifying means and thereby responsive to the control signalpulses derived from said sensing means for cyclically turning on thediac bidirectional conducting device at different phases of the positiveand negative half cycles of the alternating current supply potential tothereby derive a compensating direct current component for minimizingthe undesired direct current component flowing in the secondary windingof the supply transformer, and means for applying said compensatingdirect current component to the supply transformer by connecting theload in series circuit relationship with the diac bidirectionalconducting device and the secondary winding of the supply transformer tocontrol the load current.

7. An alternating current power circuit employing conductivitycontrolled bidirectional conducting devices including in combination analternating current supply transformer having an alternating currentsupply potential applied thereto and an undesired direct currentcomponent flowing in the secondary winding thereof which tends tosaturate the core of the supply transformer during one of the halfcycles of the alternating current supply potential, current transformersensing means operatively coupled to the supply transformer for derivingcontrol signal pulses having a polarity, magnitude and phase dependentupon the polarity, magnitude, and phase of the undesired direct currentcomponent flowing in the secondary winding of the supply transformer, anavalanche operated conductivity controlled bidirectional conductingdevice operatively coupled to the output of the current transformersensing means for deriving control signal pulses having a givenmagnitude or larger, a triac bidirectional conducting device having itsgate electrode and firing circuit means therefor operatively coupled tothe output of the avalanche operated conductivity controlledbidirectional conducting device, said triac bidirectional conductingdevice being connected in series circuit relationship with the secondarywindings of the supply transformer so that its firing circuit means isresponsive to the control signal ulses derived from the output of saidavalanche operated conductivity controlled bidirectional conductingdevice for cyclically turning on the triac bidirectional conductingdevice at different phases of the positive and negative half cycles ofthe alternating current supply potential to thereby derive acompensating direct current component for minimizing the undesireddirect current component flowing in the secondary winding of the supplytransformer, and means for applying said compensating direct currentcomponent to the supply transformer by connecting the load in seriescircuit relationship with the triac bidirectional conducting device andthe secondary winding of the supply transformer to control the loadcurrent.

8. An alternating current power circuit including in combination analternating current supply transformer having an alternating currentsupply potential applied thereacross and having an undesired directcurrent component flowing in the secondary winding thereof which tendsto saturate the core of the supply transformer on one of the half cyclesof the supply alternating current potential, current transformer sensingmeans operatively coupled to the supply transformer for deriving controlsignal pulses having a polarity, magnitude and phase dependent upon thepolarity, magnitude, and phase of the undesired direct current componentflowing in the secondary winding of the supply transformer, an avalancheoperated conductivity controlled bidirectional conducting deviceoperatively coupled to the output of the current transformer sensingmeans for deriving output control signal pulses having a given magnitudeor greater, charging circuit means operatively coupled across thesecondary winding of the supply transformer and to the output of theavalanche operated conductivity controlled bidirectional conductingdevice for developing an output firing potential dependent upon thesupply alternating current potential and the control signal pulsesderived from the current transformer sensing means by the avalancheoperated conductivity controlled bidirectional conducting device,circuit means interconnecting the avalanche 0perated conductivitycontrolled bidirectional conducting device and the charging circuitmeans for retaining the charge on the charging circuit means due to thecontrol signal pulses derived by the current transformer sensing meansover a complete cycle of the supply alternating current potential,firing circuit means including amplifying means operatively coupled tothe charging circuit means for deriving a firing pulse from the chargingcircuit means upon the potential of the charging circuit means reachinga predetermined value, a diac bidirectional conducting deviceoperatively connected in series circuit relationship across thesecondary winding of the supply transformer, said diac bidirectionalconducting device being operatively coupled across the output of thefiring circuit means for cyclically turning it on at different phases ofthe positive and negative half cycles of the alternating current supplypotential to thereby derive a compensating direct current component forminimizing the undesired direct current component flowing in thesecondary winding of the supply transformer, and means for applying saidcompensating direct current component to the supply transformer byconnecting the load in series circuit relationship with the diacbidirectional conductive device and the secondary winding of the supplytransformer to control the load current.

9. An alternating current power circuit including in combination analternating current supply transformer having an alternating currentsupply potential applied thereto and an undesired direct currentcomponent flowing in the seconday winding thereof which tends tosaturate the core of the supply transformer during one of the phases ofthe supply alternating current potential, current transformer sensingmeans operatively coupled to the supply transformer for deriving controlsignal pulses having a polarity, magnitude and phase dependent upon thedirect current component flowing in the secondary winding of the supplytransformer, an avalanche operated conductivity controlled bidirectionalconducting device operatively coupled to the output of the currenttransformer sensing means for deriving control signal pulses having agiven magnitude or greater, charging circuit means operatively coupledto the output of the avalanche operated conductivity controlledbidirectional conducting device and responsive to the alternatingcurrent supply potential for developing a firing potential that isdependent upon the value of the supply alternating current potential andthe value of the control signal pulses derived from the avalancheoperated conductivity controlled bidirectional conducting device,circuit means interconnecting the avalanche o'perated conductivitycontrolled bidirectional conducting device and the charging circuitmeans for maintaining the potential on the charging circuit meansdetermined by the value of the control signal pulses derived from thecurrent transformer sensing means over a full cycle of the alternatingcurrent supply potential, firing circuit means operatively coupled tothe charging circuit means and controlled by the charging circuit meansfor deriving a firing potential, a triac bidirectional conducting deviceoperatively connected in series circuit relationship with a load acrossthe secondary winding of the supply transformer, the control gate of thetriac bidirectional conducting device being operatively connected to thefiring circuit means whereby the phase of the turn-on time of the triacbidirectional conducting device is controlled with respect to the supplyalternating current potential to thereby obviate the undesired effectsof the direct current component flowing in the secondary winding of thesupply transformer.

10. An electric power circuit including in combination:

(a) a transformer having a set of inductively-coupled primary andsecondary windings, the primary winding being adapted to be connected toalternating voltage supply lines and one of said windings being subjectto an undesired D-C component therein;

(b) means operatively coupled to the transformer for sensing theundesired D-C component and deriving a representative output controlsignal;

(c) conductivity controlled solid-state switching means operativelycoupled with said secondary winding for conducting secondary currentduring both positive and negative half cycles of said alternatingvoltage;

(d) firing circuit means for periodically turning on said switchingmeans in timed relation to the alternating voltage cycle, said firingcircuit means being responsive to said control signal for shifting thefiring angle at which said switching means can commence conductingsecondary current of one polarity relative to the firing angle at whichsaid switching means can commence conducting secondary current of theopposite polarity in a manner to derive a compensating current componentfor minimizing the undesired D-C component in said one winding; and

(e) means for applying said compensating component to the transformer.

11. The combination set forth in claim wherein to provide thelast'mentioned means the switching means is operatively connected inseries circuit relationship with said secondary winding and with a loadcircuit and wherein current in said load circuit itself is controlled bycontrolling the switching means firing angle.

12. The combination set forth in claim 10 wherein said switching meanscomprises a bidirectional conducting device.

13. The combination set forth in claim 10 wherein said means for sensingthe undesired D-C component comprises a current transformer operativelyconnected in series circuit relationship with said one winding.

14. The combination set forth in claim 10 wherein the firing angle forsecondary current of said one polarity differs from the firing angle forsecondary current of said other polarity.

15. An electric power circuit including in combination:

(a) a transformer having a set of inductively coupled primary andsecondary windings, the primary winding being adapted to be connected toalternating voltage supply lines and one of said windings being subjectto an undesired D-C component therein;

(b) means operatively coupled to the transformer for sensing theundesired D-C component and deriving a representative output controlsignal;

(c) conductivity controlled solid state switching means operativelycoupled between said secondary winding and a load circuit for conductingload current during successive half cycles of said alternating voltage;and

(d) firing circuit means for periodically turning on said switchingmeans in timed relation to the alternating voltage cycle, said firingcircuit means being responsive to said control signal for shifting thefiring angle at which said switching means can commence conducting loadcurrent during one of said half cycles relative to the firing angle atwhich said switching means can commence conducting load current duringthe next half cycle in a manner to produce a compensating efiect forminimizing the undesired D-C component in said one winding.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 3,159,766 12/1964 Harpley 315 3,188,487 6/1965Hutson 30788.5 3,188,490 6/1965 Hoff et a1 30788.5 3,210,641 10/1965Hutson 321-46 3,242,416 3/1966 White 32322 3,263,092 7/1966 Knauss 32322LEE T. HIX, Primary Examiner G. GOLDBERG, Assistant Examiner U.S. Cl.X.R.

